Looking at IC1A: it looks like you'll be loosing channel separation compared to Rod Elliott's circuit, and it will not be the same for the inverting/non-inverting configurations. Elliott uses the inverting summer configuration, which I would recommend.

I would add a ground plane and guard traces between input and output. And why use trough hole components? SMD is usually the way to go, shorter traces.. Personally I like the BB OPA2134 a lot, but as stated already there are plenty of capable opamps out there.

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Music is art - Audio is psychoacoustics & engineering

If you decide to put it inside the plate amp, there is a reasonable chance that you could pick off the supply voltage from inside the amp. It's likely to have a regulated supply for its line-level circuitry. Some plate amps have an internal split supply like +-12V that would be perfect.

Even if it doesn't have a split supply, you could tap off whatever DC supply that it does have, regulate that down, then derive a virtual ground with a resistive divider with capacitor bypass. That would be tidier than having external wall warts and wires.

Thanks to my nice teacher I finished making to PCB for the filter. The milling machine made some mistakes in cutting the board outline and also left half of the component holes undrilled (most probably operator's fault).

Perhaps it's just a photographic artifact, but it appears that the copper plate is a bit oxidized. You might want to remove that before soldering; otherwise the solder won't flow as well. You can soak it in a vinegar/salt/water solution, then thoroughly rinse in clean water after the copper becomes bright. Google for "chemistry with pennies" for proportions.

With your layout and fabrication, there will be a fair amount of capacitance between the various opamp pins, so start with an opamp that is fairly stable; don't go for a super high bandwidth one initially.

Perhaps it's just a photographic artifact, but it appears that the copper plate is a bit oxidized. You might want to remove that before soldering; otherwise the solder won't flow as well. You can soak it in a vinegar/salt/water solution, then thoroughly rinse in clean water after the copper becomes bright. Google for "chemistry with pennies" for proportions.

With your layout and fabrication, there will be a fair amount of capacitance between the various opamp pins, so start with an opamp that is fairly stable; don't go for a super high bandwidth one initially.

Enjoy!

Hello,

I don't see any signs of oxidization on the copper layer but it has some fingerprints on it. Better to clean it with some cleaning solvent after I finish the drilling.

Also, I have measured the power supply rails on the plate amp. It has a double-sided supply of ±40 VDC. Unfortunately the maximum recommended input voltage for 78L15 is only 30 V. Maybe a simple series zener diode/transistor pre-regulator arrangement could be used to make the supply voltages suitable for 78L15.

Your pre-regulator will work, but the transistor might need some heat-sinking, depending on how much current you are drawing. I wonder if the power transformer might have additional taps for lower voltages? I would be a little surprised if +-40V is the only voltage inside the plate amp, as they usually have some lower-voltage circuitry for auto-power-on and the like. But maybe not.

There exist linear regulators that can handle more than 40V, for example MAX5023/5024, LT3012, LR12, TL783. Some semiconductor companies are pretty good about giving you a few free samples. TI is certainly very generous in that respect.

Actually, the venerable LM317 (positive) and LM337 (negative) adjustable regulators can handle an input-to-output differential of 40V. If you want to run your rails at +-15V, the differential from 40V is 25V, well within the limit.